NATURAL BRIDGES BY SOLUTION 331 



In a region of thick limestone, such as Edmonson County, Kentucky, 

 where it is said there are 100,000 miles of nndergroiind caverns, one 

 would expect natural bridges to be found in a later stage of physiographic 

 development; for, as the surface of the land is lowered by subaerial ero- 

 sion and the caverns are enlarged by subterranean erosion and solution, 

 a time must, theoretically, come when all but small portions of the cave 

 roofs will disappear, thus forming natural bridges. Caves formed in this 

 way will differ in several particulars from those described under C 1. For 

 example, as the land is lowered and the sink-holes are enlarged, the por- 

 tion of the cavern roof left standing to form the bridge will be between 

 wide basins which were originally sinks and which will clearly show their 

 origin, since the valley will widen rapidly on either side of the bridge, 

 thus showing their sink-hole origin. This consideration is well shown from 

 a study of the topographic and geologic maps of limestone regions, partic- 

 ularly those of the Kingston, Tennessee, and Bristol, Virginia, folios of 

 the U. S. Geological Survey. 



3. BY THE PARTIAL CAVING IN OF SUPERFICIAL TUNNELS 



Many bridges, initiated by solution, are formed by the partial caving 

 in of the roof of a superficial tunnel, the tunnel being formed either (1) 

 by the water of a stream which disappears in the joints of a limestone 

 stratum and appears farther down stream; or (2) by an underground 

 stream emerging as a spring (the underground stream may have a single 

 channel for but a short distance), the major portion of the roof of the 

 tunnel collapsing and leaving one or more natural bridges. 



a. Florida bridges. — There are a number of bridges of this origin in 

 Florida. Large bridges occur on the Chipola Eiver above Marianna; on 

 the Saint Marks Eiver south of Tallahassee, and on the Santa Fe Eiver 

 northeast of High Springs, the distance between the rise and sink of the 

 latter stream being two or three miles. The natural bridge on the Chipola 

 Eiver, one-quarter to one-half mile wide, is submerged during high 

 water, and a broad surface channel which crosses the one near High 

 Springs is said to carry a portion of the flood water of* the Santa Fe. 

 The breadth of the surface channel near High Springs suggests that the 

 natural bridge of the Santa Fe Eiver may have been formed by the first 

 method outlined in this paragraph. The bridge across the Chipola ap- 

 pears from the accounts to have a similar origin to the last. 



A natural bridge across Arch Creek near Miami, composed of oolitic 

 limestone (Pleistocene), more nearly satisfies our conception of a natural 

 bridge than any other in Florida. In this case the arch (plate 26, figure 

 1) stands some 12 to 15 feet above the normal surface of the stream. 



